1. Field of the Invention
The disclosure in this specification relates to a control method for controlling an engine having a precombustion chamber in which spark ignition is performed.
2. Background of the Invention
With regard to the engine having a precombustion chamber, it is been conventionally necessary to control the air fuel ratio of the gas guided into the precombustion chamber so that the ignition by the spark plug is satisfactory performed; thus, in a control method for controlling an engine having a precombustion chamber, the air fuel ratio of the air fuel mixture guide into the precombustion chamber is controlled (for instance, see Patent Reference 1). Incidentally, the air fuel ratio is the ratio of the air in mass to the fuel in mass in the air fuel mixture.
The air fuel ratio has to be kept at an appropriate level; otherwise, either air rich condition (an excessive air state) or fuel rich condition (an excessive fuel state) leads to misfiring of the engine. In addition, in view of exhaust gas emission, apart from the precombustion chamber, it is preferable to keep an appropriate air rich condition in the main combustion chamber.
FIG. 5 shows an example of a conventional engine of a precombustion chamber type. As shown in FIG. 5, a main combustion chamber S1 is formed over a piston 1; a precombustion chamber-forming piece 3 of a tapered shape is arranged on the head side of a cylinder 2. In the precombustion chamber-forming piece 3, a precombustion chamber S2 is formed. On the upper side of the precombustion chamber-forming piece 3, a spark (ignition) plug 4 is provided for igniting the air fuel-gas mixture in the precombustion chamber S2. Further, over the precombustion chamber-forming piece 3, a fuel gas supply pipe 5 for feeding the fuel gas is provided. On a part way of the fuel gas supply pipe 5, a precombustion chamber fuel supply valve 6 for regulating the flow amount of the fuel is provided.
Further, the space in the main combustion chamber on the head side of the cylinder 2 communicates with an air charging pipe 7 for charging fuel gas (or air fuel-gas mixture) into the main combustion chamber S1. An air intake valve 8 is provided at the boundary between the air charging pipe 7 and the cylinder 2, the boundary demarcating the space inside of the main combustion chamber S1 and the passage formed by the air charging pipe 7. On a part way of the air charging pipe 7, a fuel gas supply pipe 9 for making fuel gas stream into the air charging pipe 7 is connected to the air charging pipe 7. In addition, on a part way of the fuel gas supply pipe 9, a main fuel supply valve 10 for regulating the flow rate of the fuel toward the main combustion chamber S1 is provided. Further, the fuel gas supply pipe 9 communicates with the precombustion chamber S2 as well as communicates with the fuel gas supply pipe 5 connected to the precombustion chamber S2; namely, a fuel gas pipe branches into two passages (i.e. the fuel gas supply pipes 9 and 5); thereby, the fuel gas supply pipe 9 supplies fuel to the main combustion chamber S1, while the fuel gas supply pipe 5 supplies fuel to the precombustion chamber S2.
The air fuel-gas mixture streaming into the main combustion chamber S1 through the air charging pipe 7 burns in the main chamber 10, and produces combustion gas; the produced combustion gas is discharged as exhaust gas from the main combustion chamber 10 through an exhaust (gas discharging) pipe 11. An exhaust valve 12 is provided at the boundary between the air charging pipe 7 and the cylinder 2, the boundary demarcating the space inside of the main combustion chamber S1 and the passage formed by the exhaust (gas discharging) pipe 12.
The various valves such as the fuel supply valves 6 and 10 are controlled by ECU (an engine control unit) 13.
On the other hand, a part of lean air fuel mixture guided into the main combustion chamber S1 for main combustion streams into the precombustion chamber S2 via the main combustion chamber. In order to obtain an appropriate air fuel ratio of the air fuel-gas mixture guided into the precombustion chamber, fuel gas is supplied to the precombustion chamber via the precombustion chamber fuel supply valve 6. Hence, various approaches to regulate the appropriate air fuel ratio of the air fuel-gas mixture guided into the precombustion chamber S2 have been conventionally studied and developed (e.g. Patent References 2 and 3)
In one example of the studied and developed approaches for controlling the engines having a precombustion chamber, the fuel-gas flow toward the precombustion chamber is controlled in response to the engine operation conditions such as engine load conditions. In such an approach, however, when the engine load or the engine speed is increased abruptly, the air fuel ratio of the air fuel-gas mixture guided into the main combustion chamber may be transiently in shortage (i.e. the air fuel-gas mixture is transiently placed in a fuel rich condition); accordingly, the air fuel ratio of the air fuel-gas mixture guided into the precombustion chamber is placed in a fuel rich condition. Thus, misfiring may be caused in the precombustion chamber.
In the next place, the types regarding misfiring (i.e. how misfiring occurs) are hereby explained.
FIG. 6(a) shows how the air excess ratio (explained in the later description regarding embodiments) of the air fuel mixture in the precombustion chamber changes in response to the engine speed change. In FIG. 6(a), the solid line is a locus regarding the response of the air excess ratio of the air fuel mixture in the precombustion chamber with regard to the engine speed in a case where the engine speed is quasi-statically increased; thereby, the air excess ratio of the air fuel mixture in the precombustion chamber is not decreased and is kept in an equilibrium state, even though the engine speed is increased. In a case where the engine speeds up and the engine speed is abruptly increased, however, the quasi-static line connecting the points A and B bends downward so as to form an arc shape as shown with the broken line in FIG. 6(a). Even in this case, the broken line is out of the misfiring zone in FIG. 6(a). However, in a case where the engine speed is further abruptly increased, the locus regarding the response of the air excess ratio against the engine speed forms a dash-dot line connecting the points A and C so that the air excess ratio reaches the misfiring zone.
Further, on the basis of FIG. 6(a), FIG. 6(d) as well as FIG. 6(e) is explained. FIG. 6(e) shows the relation between the fuel amount supplied into the main combustion chamber and the engine speed; thereby, along the solid line (in a case where the engine speed is quasi-statically increased), the fuel amount in the main combustion chamber is linearly increased in response to the increase of the engine speed. In addition, along the broken line (namely, in a case where the engine speed is abruptly increased) in FIG. 6(e), the fuel amount in the main combustion chamber is further increased than in the case where the engine speed is quasi-statically increased; accordingly, the air excess ratio of the air fuel mixture in the main combustion chamber is decreased. As a result, the fuel amount in the air fuel mixture that streams into the precombustion chamber via the main combustion chamber is increased. Therefore, in this situation of the increased fuel amount in the precombustion chamber, if the fuel amount into the precombustion chamber is linearly controlled as shown in FIG. 6(b), the air excess ratio of the air fuel mixture in the precombustion chamber is decreased as shown in FIG. 6(a). Further, when the engine abruptly speeds up (namely, in a case of the dash-dot line), the misfiring is caused at the point C as described above, and the engine speed is decreased as shown in FIG. 6(e).
In a case where a misfiring is caused, the engine speed is decreased as described above; thus, in a case where the feedback control regarding the fuel amount supplied to the main combustion chamber is performed with regard to the engine speed, the amount of the fuel supplied to the main combustion chamber is increased in response to the decrease of the engine speed; thus, the misfiring is further inclined to happen, as shown in FIG. 6(a).